The present invention relates to a method of producing thermotropic liquid crystalline polyester (hereinafter referred to as LC polyester) and, more particularly, to a method of producing LC polyester having good heat resistance and good fluidity and uniformity.
Along with recent developments in the fields of electric, electronic and car industries, demands for plastics of higher capacity have increased, and a variety of plastics have been developed and provided on the market. Among them, a group of polymer compounds known as thermotropic liquid crystalline polymers, which exhibit optical anisotropy when melted, and in which the molecular chains are aligned in parallel, have been catching attention since they show good molding processability and improve the mechanical properties of the molded articles.
A typical LC polyester is a copolymer polyester made of only ester linkages of polyethylene terephthalate units and para hydroxy benzoic acid units (W. J. Jackson et al, J. Polym. Sci. Polym. Chem. Ed. Vol. 14, p. 2043 (1976), U.S. Pat. No. 3,804,805, and Japanese Patent Laid-Open Publication No. 51-8395).
In the above mentioned Japanese Patent Laid-Open Publication No. 51-8395, a method of producing a LC polyester by mixing polyethylene terephtharate units and para acetoxy benzoic acid units (hereinafter referred to as p-ABA) and heating them at 240.degree.-300.degree. C. to carry out polymerization, is described. It is known that increasing the content of p-ABA results in improvement of the heat resistance of the produced LC polyester, however, in this method, when the p-ABA component holds more than 75 mol % of the total aromatic group of the polymer, such a polymer considered to be produced by block polymerization of p-ABA, exists as an insoluble, infusible matter (comparative example 1), and the resulting LC polyester shows inferior fluidity, and lowered moldability. The generation of such foreign matter is also confirmed by the existence of a large amount of insoluble substances, when such a polymer is dissolved in a good solvent.